The present invention relates to a control apparatus for a cycloconverter and, more particularly, to a control apparatus for a cycloconverter which improves an input power factor of the cycloconverter.
A cycloconverter is an apparatus for directly converting AC power with a first frequency into another AC power with a second frequency. As well known, a cycloconverter having the combination of a plurality of bridge converters of the natural commutation type has been used as a frequency converter with a large capacity, because it is stably operable under quick-response control. The input power factor of the cycloconverter is inferior to that of the DC converter. Thus, the input to the cycloconverter contains a large amount of reactive power component. Since a voltage across a load connected to the output terminal of the cycloconverter generally takes a sinusoidal waveform, the AC input voltage to the cycloconverter is selected so that the output voltage of a bridge converter (voltage between a common connection point of the bridge converters and a load connecting terminal of the corresponding bridge converter) can reach a crest value of the sinusoidal load voltage. Even if the crest value of the load voltage is equal to the maximum output voltage of the cycloconverter, voltage values in most regions of one cycle of a load voltage is smaller than the maximum output voltage value of the cycloconverter.
A plurality of SCR's (silicon controlled rectifiers) of each of the bridge converters, which constitute the cycloconverter, receive at the gates control signals supplied from a control apparatus for the cycloconverter and undergoes the control of the control signals. It is assumed that the control delay angle of each SCR is .alpha.. It is also assumed that the output frequency of the cycloconverter is zero, that is, a constant DC current flows from the cycloconverter. Under this ideal operating condition of the cycloconverter, the input power factor of the cycloconverter is cos .alpha., the DC output voltage of the cycloconverter is proportional to cos .alpha., and the reactive input power to the cycloconverter is proportional to sin .alpha.. In order to improve the input power factor to the cycloconverter by reducing the reactive input power to the respective bridge converters, each bridge converter must be controlled, therefore, so as to produce a positive or negative output voltage, which is as large as possible, over tolerable widest possible part of one cycle period of the load phase voltage.
The input power factor of the cycloconverter depends largely on a voltage conversion ratio .lambda.(.lambda.=crest value of the load voltage/maximum value of an ideal output voltage of a cycloconverter, i.e. the output voltage of the cycloconverter when the control delay angle .alpha. of each SCR is zero), and a power factor of the load itself. The larger the voltage conversion ratio and the power factor of the load itself, the better the input power factor of the cycloconverter is improved. Disregarding this fact, however, the output voltage of each bridge converter in the conventional cycloconverter is so controlled as to have a sinusoidal waveform. Therefore, the input power factor of the cycloconverter was lower than that of a DC converter.